Specifying
Steam Generators,Waste Heat Boilers V.GanapathySteam
generators and waste heat boilers are critical equipment in any plant and
have a life of 20 to 30 years. Unfortunately specifications developed by
consultants/engineers often lack clarity and basic process/operating cost
information to design the optimum system. Often operating costs are
ignored and equipment are purchased based solely on initial costs,which
may be appealing at first but in the long run,it is the plant which loses.
Operating costs for Waste heat boilers arise due to gas pressure drop and
fuel consumption if fired.A good design optimizes the initial and the operating
costs so that the life cycle cost of owning and operating the equipment
is minimized. For steam generators, fan power consumption,fuel consumption
and maintenance costs (refractory/superheater failures etc) have to be
evaluated in addition to initial costs. The completely water cooled furnace
design in packaged boilers for example results in lower maintenance costs
(compared to refractory filled boilers with associated maintenance and startup costs),though the initial
cost may be slightly higher.However over a period of time additional investment
in these features pays off. Secondary heat recovery in HRSG systems is
not often looked into.See
the article on Methods to Improve HRSG Efficiency .
Several
specifications have hardly a page or so on process information,while they
are filled with hundreds of pages on aspects such as painting,legal mumbo
jumbo,reservoir water table etc etc! Here are a few pointers from process
viewpoint which I feel should be included as a minimum by engineers involved
in developing specifcations for packaged steam generators and waste heat
boilers.

Waste
Heat boiler specificationsThe
following discussions pertain to waste heat boilers in incineration plants,chemical
plants,gas turbine cogeneration systems,refineries and boilers recovering
energy from various flue gas sources.1.Describe
the application. Where is the gas coming from? How is it generated? Process
or flow diagrams help.2.Gas
flow in mass units such as lb/h and NOT in volumetric units such as
acfm,scfm etc. Also provide the flue gas analysis such as % volume CO2,H2O,N2,O2,SO2,Hcl
etc. This helps to determine if low or high temperature corrosion problems
are likely. Gas specific heat and hence boiler duty,steam generation and
temperature profiles are also impacted by gas analysis. Gas pressure is
also important. High gas pressure (say above 2 to 3 psig) may call for
special casing designs to contain the gas.3.Nature
of the gas,whether clean or dirty or slagging. The boiler design is very
much impacted by this. For clean gases as in gas turbine HRSGs or fume
incineration heat recovery boilers,extended surfaces can be used to make
the boiler compact. If the gas is dirty bare tubes may have to be used,making
the boiler expensive and heavy. If slagging,a radiant furnace may be required
to cool the gases to below slagging levels before it enters the convection
bank. Also soot blowers or other cleaning systems may be required.4.Suggested
gas pressure drop.. This helps one to compare various designs. In small
HRSGs 3 to 6 in wc is typical,while in large HRSGS it could range from
6 to 12 in wc. 5.Suggested
exit gas temperature or duty or steam output. This may not always be possible
initially. In gas turbine HRSGs and some incineration applications, a simulation
using the software "HRSGS" may be performed to arrive at gas/steam temperature
profiles. In incineration plants, a scrubber may be used behind the HRSG
and hence the exit gas temperature may not exceed say 450 -500 F. In some
applications such as sulfuric acid plants or hydrogen plants,the exit gas
temperature from the Waste Heat Boiler is dependent on the process downstream
of the boiler. 6.Emission
levels of NOx,CO desired. This may require catalysts for NOx,CO reduction
located within the HRSG surfaces at the optimum gas temperatures. 7.Fuel
analysis in case auxilliary firing is required in the HRSG.8.Steam
purity requirements in case steam is used for injection in gas turbines
or steam turbines. 9.Avoid
specifying surface areas. Surface areas can be misleading. Since energy
transferred Q=USDT(U=overall heat transfer coefficient,S=surface area and
DT=log-mean temperature difference),unless you know how to evaluate U for
each surface,it is not a good idea to look at S alone. By using different
velocities,optimum tube diameters or fin configuration,it is possible to
transfer more energy with lesser surface area..Redistribution of energy
among radant and convective surfaces can also distort the picture.By using
a lower transverse pitch for the tubes and with the same surface area,one
can transfer more energy at higher gas pressure drop..and vice versa.So
don't select a boiler simply because it has more surface area..With
finned tubes,by using higher fin densities you can have a higher surface
area (and a lower U) transferring the same duty.The tube wall temperatures
and fin tip temperatures can run hotter with higher fin densities.(higher
ratio of external to internal tube surface). Heat flux inside tubes will
also be much higher if a large ratio of external to internal surface area
is used can lead to DNB problems or tube failures due to overheating...10.While
using interstage attemperation for superheated steam temperature control,be
sure to mention if demineralized water is available. If not then other
systems have to be considered such as use of condensate spray obtained
by condensing saturated steam..More
detailed discussions may be found in my "Waste Heat Boiler Deskbook".

Specifying
Steam Generators1.Provide
basic steam parameters,fuel analysis,space limitations if any and whether
the boiler is for continous or standby operation. This may impact the need
for an efficient or inefficient boiler. 2.While
specifying steam generators,be sure to mention if deaeration steam is to
be included in the steam output of the boiler..this can be a substantial
amount if make up is quite large.. 3.It
is not a good idea to select a large boiler capacity and operate it at
very low loads..this is moreso if you have a radiant superheater..At low
loads,the flow distribution through the tubes and outside the tubes are
unpredictable..(if the steam pressure drop is 50 psi at 100 % load,it is
hardly 3 psi at 25 % load)..hence flow maldistribution can cause overheating
and possible tube failures..With convective superheaters,the prblem may
not be as severe but it is better to avoid low loads..also the fan operating
point has to be checked at low loads,whether it is capable of stable operation....4.State
emission levels of CO and NOx upfront..You cannot buy the boiler first
and then check for emissions later..If flue gas recirculation is to be
used to meet the new NOX levels,the existing fan system may not be adequete..also
the superheater performance and boiler efficiency will be affected. 5.Surface
areas,again,should NOT be specified. There are several ways of arriving
at the same overall performance (by transferring different amounts of energy
in the furnace,convection section and economizer). One should look for
overall efficiency,fan power consumption and maintenance costs.6.If
steam temperature is controlled using spray attemperator,then demineralised
water should be provided or condensate may have to be used.7.Specifying
volumetric heat release rates is not meaningful in gas or oil fired steam
generators,though many consultants do this. This is more applicable to
fuels which are difficult to burn such as solid fuels or fuels which require
some residence time for combustion. Area heat release rate is more meaningful
as it relates to heat flux and DNB concerns.8.Include
cost of fuel,electricity in the specifications and hours of operation per
year,so that the boiler designer can optimize the design considering both
the initial and the operating costs.This
applies to both steam generators and waste heat boilers.